1. Field of the Invention
The present invention generally relates to: micro optical systems applying micro machining techniques; and image forming apparatuses such as digital copying machines and laser printers, and more particularly, to: an optical scanning apparatus using a beam-supported-type vibration mirror driven by an electrostatic force; an optical scanning apparatus that can be applied to, for example, an optical-scanning-type barcode reader and an in-vehicle laser radar; and an image forming apparatus using such an optical scanning apparatus.
2. Description of the Related Art
The optical scanning apparatus using a beam-supported-type vibration mirror driven by electrostatic force is a promising candidate for an optical writing apparatus of an image forming apparatus such as a digital copying machine and a laser printer, and for an optical reading apparatus such as a barcode reader and a scanner.
“Silicon Torsional Scanning Mirror”, Kurt E. Petersen, IBM Journal of Research and Development Vol. 24, 1980, pages 631-637 discloses a beam-supported-type vibration mirror that causes a mirror substrate supported by two beams provided on the same line to perform reciprocating motion by twisting the two beams with electrostatic force exerted between the mirror substrate and electrodes provided at positions opposing the mirror substrate, while using the two beams as the rotation axis. The vibration mirror manufactured by using a micro machining technique has a simple structure compared to an optical scanning apparatus configured to rotate a polygon mirror by using a motor, and can be integrally formed in a semiconductor process. Thus, the size of the vibration mirror can be easily reduced and manufacturing costs thereof are low. In addition, since a polygon mirror uses a plurality of mirror surfaces, there is a problem of variation in accuracy of each of the mirror surfaces. However, the vibration mirror having only a single mirror does not have such a problem. Further, it is possible for the vibration mirror to easily correspond to high-speed scanning performed by reciprocating scanning.
Various electrostatically-actuated vibration mirrors are known such as: an electrostatically-actuated vibration mirror that decreases the rigidity of a beam by forming the beam into an S-shape so as to achieve a large swing angle with a small driving force (refer to Japanese Patent Publication No. 2924200, for example); an electrostatic vibration mirror having a beam whose thickness is thinner than those of a mirror substrate and a frame substrate (refer to Japanese Laid-Open Patent Application No. 7-92409, for example); an electrostatically-actuated vibration mirror in which driving electrodes are arranged at a position that does not overlap with swinging directions of a mirror part (refer to Japanese Patent Publication No. 3011144 and “An Electrostatically Excited 2D-Micro-Scanning-Mirror with an In-Plane Configuration of the Driving Electrodes”, Harald Schenk, The 13th Annual International Conference on MEMS 2000, pages 473-478, for example); an electrostatically-actuated vibration mirror that reduces a driving voltage without changing a swing angle of a mirror by providing a driving electrode in a slanted manner with respect to the center position of the swing of the mirror (refer to “Fabrication, Simulation and Experiment of a Rotating Electrostatic Silicon Mirror with Large Angular Deflection”, Camon Henri, The 13th Annual International Conference on MEMS 2000, pages 645-650, for example); and an electrostatically-actuated vibration mirror having an electrode for actuation in addition to a driving electrode (refer to Japanese Laid-Open Patent Application No. 2002-267995).
Conventionally, there is a vibration mirror that causes a mirror substrate to perform reciprocating motion while using as the rotation axis two beams provided on the same line to support a mirror substrate at two opposing sides thereof by driving the mirror substrate with electrostatic force exerted between two movable electrodes provided on the other opposing two sides of the mirror substrate and driving electrodes opposing to the movable electrodes. Such a vibration mirror is driven to perform reciprocating motion at a resonance point. However, as can be seen from FIG. 14 showing measurement results, the above-mentioned vibration mirror has a problem in that the swing angle (vibration amplitude) of the mirror substrate is significantly varied when environmental temperature is changed. The problem is caused since the resonance point of a vibrating system of a vibration mirror varies depending on environmental temperature and the variation of the resonance point significantly affects the swing angle of the mirror substrate.
A description is given below of the resonance point of such a vibration mirror and variation of the resonance point due to change in environmental temperature. The resonance point may be approximated by the following equation (1), where kθ represents a torsional elastic coefficient of a beam, and I represents a moment of inertia of the mirror substrate.f=½π√{square root over ( )}(k/I)  (1)
The torsional elastic coefficient kθ is given by the following equation (2) where c represents the width of the beam, t represents the height of the beam, and L represents the length of the beam. It should be noted that β represents a modulus of section, E represents Young's modulus, and ν represents Poisson's ratio.kθ=β·t·c3·E/L(1+ν)  (2)The Young's modulus E at a temperature tmp is obtained by the following equation (3), provided that the temperature coefficient is Δht.E=E0(1−Δht*tmp)  (3)It should be noted that E0 is given by the following equation (4).E0=1.9e+12(dyne/cm2), Δht=75e−6/° C.  (4)
From the above equations (1) through (4), it is understood that the Young's modulus E is decreased in proportion to the increase in the temperature tmp. Accordingly, it is understood that the resonance point falls when the temperature tmp is increased.
In order to reduce variation of the swing angle caused by change in environmental temperature, similarly to an optical scanner driven by a piezoelectric element disclosed in, for example, Japanese Patent Publication No. 2981600, it is possible to apply a mechanism in which an electric resistive element serving as a heater element is provided, and variation of Young's modulus is suppressed by increasing or decreasing the heat value of the electric resistive element. However, it is undesirable in terms of reliability to provide an electric resistive element in a beam that is elastically deformed. Additionally, when the electric resistive element is provided, the manufacturing process of a vibration mirror is complicated, and additional means are required for controlling a current of the electric resistive element, which are problems in terms of costs.
Conventional optical scanning apparatuses use a polygon mirror or a Galvano mirror as a deflector that scans an optical beam. In order to achieve a higher resolution image and high-speed printing, it is necessary to further increase the moving speed of the mirror, which may present problems in durability of a bearing supporting the mirror, heat generation due to windage loss of the mirror, and noise, for example. Thus, there is a limit for such conventional optical scanning apparatuses to perform high-speed scanning.
On the other hand, recently, studies have been made on optical deflectors using micro machining techniques, and methods have been proposed that integrally form a vibration mirror and a beam supporting the vibration mirror from a Si substrate (refer to Japanese Patent Publications No. 2924200 and No. 3011144, for example). According to the proposed techniques, since reciprocating vibration is performed by using resonance, there is an advantage in that noise is low despite that a high-speed operation is performed. Additionally, it is possible to reduce power consumption since a driving force for rotating the vibration mirror is small.
By using a vibration mirror as mentioned above, compared to the conventional methods that use a polygon mirror, it is possible to provide an optical scanning apparatus having a reduced size and consuming less power. However, the swing angle of the vibration mirror is small, and there is a limit to the size of a reflection surface. Hence, a method has been proposed in which a plurality of optical scanning apparatuses having short optical path lengths are arranged in parallel, thereby diving an image to be constituted in the main scanning direction, reducing respective recording lengths, and splicing them together (refer to Japanese Laid-Open Patent Application No. 2001-228428).
However, when using a plurality of vibration mirrors and scanning in a divided manner as mentioned above, variation in the resonance frequency of each of the vibration mirrors may become a major problem. This is because when the variation in the resonance frequency is large, it is difficult or impossible to drive the plurality of vibration mirrors with a common driving frequency. It should be noted that the span of adjustable range of the swing angle of a mirror is extremely small.
Variation in a resonance frequency may be caused by the following factors.
(i) variation in processing during production
(ii) variation due to change in environmental temperature and/or humidity
(iii) variation in ambient pressure (when used in the atmosphere)
Accordingly, the above-mentioned problem cannot be avoided, and it is necessary to select one of the following options: for example, driving the vibration mirrors with respective driving frequencies corresponding to respective resonance frequencies; selecting and using those vibration mirrors that fall within a predetermined range, and driving the vibration mirrors with the same driving frequency, which is undesirable in terms of process yield; and adding a complicated driving system whereby controlling and driving the vibration mirrors.
Countermeasures for variation in a resonance frequency due to variation in processing (the above item (i)) include a method that, in a manufacturing process of a vibration mirror, after the vibration mirror and a torsion beam are formed, performs etching or depositing on the vibration mirror and/or the torsion beam so as to vary the mass thereof (generally referred to as “trimming”) while driving the vibration mirror, thereby adjusting the resonance frequency to fall within a predetermined range (refer to Japanese Laid-Open Patent Applications No. 2002-40353, No. 2002-40355, No. 2002-228965).
However, since the above-mentioned method performs the adjustment in the middle of the manufacturing process, there are problems in that a shift tends to occur if adjustment is not performed in prospect of a difference between the resonance frequencies before and after completion of the vibration mirror, and it is difficult or impossible to correspond to variation in the resonance frequency under an environment subjected to the above items (ii) and/or (iii).
In addition, the resonance frequency of a vibration mirror is fundamentally determined to a unique value by the rigidity of an elastic member (torsion beam) and the inertia of the vibration mirror. Hence, countermeasures for variation in the resonance frequency due to temperature change under an environment subjected to the above item (ii) include a method that provides a heater (resistance heating) to a torsion beam, which is an elastic member, and maintains the temperature of the elastic member at a constant value, thereby suppressing rigidity variation due to change in environmental temperature, i.e., frequency variation (refer to Japanese Laid-Open Patent Application No. 9-197334, for example).
However, there are problems in the above-mentioned method. For example, since the above-mentioned method provides the heater, it is inevitable to avoid an increase in the manufacturing costs for the heater. In addition, since electricity is continuously conducted to the heater, power consumption also is increased. Further, since the temperature of the elastic member is controlled by heat generation of the heater, there is a problem in that it is difficult or impossible to correspond to decrease in environmental temperature in a positive manner.
Additionally, the countermeasures also include a method in which a vibration mirror is bonded and fixed to a base member having a thermal expansion coefficient different from that of the vibration mirror, and rigidity variation in an elastic member is canceled out by using a stress created due to the difference between the thermal expansion coefficients of the vibration mirror and the base member, which difference is generated due to temperature change, thereby suppressing frequency variation (refer to Japanese Laid-Open Patent Application No. 2002-321195, for example).
However, with the above-mentioned method, the stress is generated in various ways depending on the structure, materials, and bonding methods. Hence, it is doubtful whether it is possible to design a vibration mirror such that the above-mentioned stress is effectively generated, and whether the stress is effectively generated as designed, when considering errors inevitably introduced during the production of the vibration mirror.
Generally, a structure is used in which the vibration space of a vibration mirror is sealed with respect to variation in a resonance frequency in the case where the above item (iii) exists.
Further, countermeasures for the above items (i) through (iii) include a method that uses a driving circuit constituted by a feedback circuit having a relatively simple structure, thereby positively driving a vibration mirror at a resonance frequency (refer to Japanese Laid-Open Patent Application No. 2002-277809, for example). However, the method drives the vibration mirror with an electromagnetic force. In the method, a coil formed on the vibration mirror for conducting a driving current is commonly used, and feedback is given by detecting a counter electromotive force. Thus, there is, for example, a limitation that the method is inapplicable to a vibration mirror that uses a driving force other than an electromagnetic force.